1. Field of the Invention
The present invention relates to a display device for conducting a display operation using four primary colors.
2. Description of the Related Art
A color display device such as a color TV monitor or a color display monitor represents colors usually by adding together the three primary colors of red (R), green (G) and blue (b). Thus, each pixel in a color display device (such as a liquid crystal display device) has red, green and blue sub-pixels for these three primary colors of RGB. By controlling the luminances of these red, green and blue sub-pixels to desired values, a variety of colors can be represented.
The luminance of each sub-pixel varies within the range from the one corresponding to the lowest gray scale level thereof (e.g., gray scale level 0) through the one corresponding to the highest gray scale level (e.g., gray scale level 255). If all of these sub-pixels, namely, the red, green and blue sub-pixels, have the minimum luminance, the color represented by the pixel is black. Conversely, if all of these sub-pixels have the maximum luminance, the color represented by the pixel is white.
FIG. 16 schematically illustrates a pixel in a conventional LCD. As shown in FIG. 16, a single pixel has three sub-pixels (i.e., red, green and blue sub-pixels). Strips of red, green and blue sub-pixels are arranged in stripes in this order. These red, green and blue sub-pixels are formed by defining three sub-pixel regions in a single pixel region on a color filter (not shown).
The following Table 1 shows exemplary XYZ color system values of colors represented by pixels in a conventional LCD:
TABLE 1RedGreenBlueWhiteX94.775.840.1210.5Y48.2155.714.6218.5Z3.519.1231.2253.8x0.6460.3020.1400.308y0.3290.6220.0510.320z0.0240.0760.8090.372Color temperature = 6800 K
In Table 1, “Red” shows the XYZ color system values of a color represented by a pixel in a situation where the red sub-pixel has the maximum luminance but the green and blue sub-pixels have the minimum luminance. In the same way, “Green” shows the XYZ color system values of a color represented by a pixel in a situation where the green sub-pixel has the maximum luminance but the red and blue sub-pixels have the minimum luminance. “Blue” shows the XYZ color system values of a color represented by a pixel in a situation where the blue sub-pixel has the maximum luminance but the red and green sub-pixels have the minimum luminance. And “White” shows the XYZ color system values of white represented by a pixel in a situation where the red, green and blue sub-pixels all have the maximum luminance.
Also, X, Y, Z, x, y and z of Table 1 are the X, Y, Z, x, y and z according to the XYZ color system. It should be noted that X, Y and Z are rounded off to the first decimal place, while x, y and z are rounded off to the third decimal place. As is apparent to those skilled in the art, according to the XYZ color system, x=X/(X+Y+Z), y=Y/(X+Y+Z), and z=Z/(X+Y+Z), where x and y are also called “chromaticity” and Y corresponds to the lightness. The results shown in Table 1 were obtained using a backlight, a liquid crystal layer, and color filters with the following properties.
FIG. 17 is a graph showing the spectrum of the light emitted from the backlight (not shown) of a conventional LCD. This backlight emits white light with chromaticities (xy)=(0.31, 0.30). FIG. 18 is a graph showing the wavelength-transmitted light intensity characteristic of the liquid crystal layer of a conventional LCD.
FIG. 19 is a graph showing the wavelength dependences of respective color filters for red, green and blue sub-pixels. In FIG. 19, the curve “Red” shows how the transmittance changes with the wavelength in a color filter for the red sub-pixel, the curve “Green” shows how the transmittance changes with the wavelength in a color filter for the green sub-pixel, and the curve “Blue” shows how the transmittance changes with the wavelength in a color filter for the blue sub-pixel.
FIG. 20 schematically shows the luminances of red, green and blue sub-pixels when a conventional LCD represents white. In FIG. 20, R, G and B denote the red, green and blue sub-pixels, respectively. When white is represented, the red, green and blue sub-pixels have the maximum luminance.
In the conventional LCD, the X of the white is the sum of the respective X of Red, Green and Blue. In the same way, the Y of the white is the sum of the respective Y of Red, Green and Blue. And the Z of the white is the sum of the respective Z of Red, Green and Blue. Thus, as shown in Table 1, the X, Y and Z of the white are 210.5 (=94.7+75.8+40.1), 218.5 (=48.2+155.7+14.6) and 253.8 (=3.5+19.1+231.2), respectively. In this case, the x, y and z are 0.308, 0.320 and 0.372, respectively. The color temperature of the conventional LCD is 6800 K as shown in Table 1.
FIG. 21 is a graph showing a chromaticity diagram of the XYZ color system. In FIG. 21, the abscissa and ordinate of this graph represent x and y coordinates in the XYZ color system. The graph of FIG. 21 shows a spectrum locus and dominant wavelengths. The spectrum locus shows the relation between the color temperature of the blackbody locus and the chromaticity. As the temperature of the blackbody (which is a perfect radiator) is increased, the light emitted from the blackbody changes its colors. In this case, the absolute temperature T [K] of the blackbody is called a “color temperature”. In general, white of a display device is represented by the color temperature.
Such display devices that use the three primary colors have been used extensively so far. Recently, however, a display device for an additive mixture of four primary colors was also proposed. Such a display device further increases the number of primary colors used by adding Ye to the three primary colors of RGB, thereby expanding the color reproduction range (see Patent Documents Nos. 1 to 3, for example).                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2001-306023        Patent Document No. 2: PCT International Application Japanese National Phase Publication No. 2004-529396        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2001-209047        
FIG. 22 schematically illustrates a pixel in the display devices disclosed in Patent Documents Nos. 1, 2 and 3, for example. As shown in FIG. 22, the single pixel has four sub-pixels (namely, red, green, blue and yellow sub-pixels), which are arranged in a mosaic pattern.
The color filters of such a display device has wavelength dependences such as those shown in FIG. 23. As in FIG. 23, the curves “Red”, “Green” and “Blue” show how the transmittance changes with the wavelength in color filters for the red, green and blue sub-pixels, respectively, just like as shown in FIG. 19. And the curve “Yellow” shown in FIG. 23 shows how the transmittance changes with the wavelength in a color filter for the yellow sub-pixel. Supposing this display device uses a backlight with the spectrum shown in FIG. 17 and uses a liquid crystal layer with the wavelength-transmitted light intensity characteristic such as that shown in FIG. 18, the display device conducts a display operation in the following manner.
The following Table 2 shows exemplary XYZ color system values of colors represented by pixels in such a display device:
TABLE 2RedGreenBlueYellowWhiteX63.951.227.1179.0321.1Y32.6105.19.8195.7343.3Z2.412.9156.111.6182.9x0.6460.3020.1400.4630.379y0.3290.6220.0510.5070.405z0.0240.0760.8090.0300.216Color temperature = 4200 K
In Table 2, “Yellow” shows the XYZ color system values of a color represented by a pixel in a situation where the yellow sub-pixel has the maximum luminance. In this case, the red, green and blue sub-pixels of an LCD that uses these four primary colors have the similar configuration as their counterparts of the LCD shown in FIG. 16 that uses the three primary colors except that the area of each sub-pixel in the former LCD is three-quarters of that of its counterpart in the latter LCD. That is why the X, Y and Z values of Red, Green and Blue in Table 2 are three-quarters of those of their counterparts in Table 1. Just like the display device that uses the three primary colors, this display device also sets the luminances of all sub-pixels to be maximum in representing white.
FIG. 24 schematically shows the luminances of red, green, blue and yellow sub-pixels when this display device represents white. In FIG. 24, R, G, B and Ye denote the red, green, blue and yellow sub-pixels, respectively. When white is represented, the red, green, blue and yellow sub-pixels have the maximum luminance.
In this case, the X of the white is the sum of the respective X of Red, Green, Blue and Yellow. In the same way, the Y of the white is the sum of the respective Y of Red, Green, Blue and Yellow. And the Z of the white is the sum of the respective Z of Red, Green, Blue and Yellow.
Thus, the X, Y and Z of the white are 321.1 (=63.9+51.2+27.1+179.0), 343.3 (=32.6+105.1+9.8+195.7), and 182.9 (=2.4+12.9+156.1+11.6), respectively. In this case, the x, y and z are 0.379, 0.405 and 0.216, respectively. The color temperature of this display device is 4200 K as shown in Table 2. Look at FIG. 21 again, and it can be seen that the display device using the four primary colors has a lower color temperature than its counterpart using the three primary colors, and therefore, represents yellowish white. As a result, the display quality declines in the display device that uses the four primary colors.